Power regulation schematic for 12V battery?

[blackietje]

Hi,

So for my project i'm up for the first task of making a power regulation board for 2x 12V DC motors.
I've searched the forum a little and found a 24V schematic (from Soeren): http://That.Homepage.dk/PDF/24V+5V_PSU.pdf
Since i have to power 2x 12V motors i'm wondering what i have to change to this schematic to get a good power regulator.
Some things i don't understand about the schematic is the inductor L1, and the K1  with 2 connections to mass and battery regulation. What are these for?
The 24V from battery doesn't get regulated in this schematic for the motor?

I have 2 LM350T switching regulators at my disposition. Can i use a normal 12V regulator (as my battery is 12V), and for the 12=>5 the switching one?
I'm also a bit confused about the capacitors 10mF in the original power regulation scheme from this website means 10µ ? How must i adapt these values to account for 12V
The 2 motors draw 2,8 A each and have a stall current of 6A. Powering the motors is essential because my other circuits are allready regulated.

any help would be greatly appreciated

[Admin]

That schematic is *not* what you want.

It's just a battery monitor plus noise filter built around a Dimension Engineering switching regulator.

The 2 motors draw 2,8 A each and have a stall current of 6A.

The biggest switching regulator DE has, using your 12V motors, will only let you go up to 2A:
http://dimensionengineering.com/DE-SWADJ3.htm
. . . but you need 12A. So no good.


The #1 solution would be to use a 12V battery.

[blackietje]

Well i will use a 12V lead-acid battery, but how can i regulate the power then?
I've been told that a fully charged battery will be at about 14-15V, so what schematic or solution should i use? Maybe you thought i wanted to use the 24V for 2x12v motors, but that was not my intention. I just want a schematic for a 12V battery, but i'm a bit confused on how to do this. Just a standard 12V regulator with heatsink?

I have 2 LM350T switching regulators at my disposition. Can i use a normal 12V regulator (as my battery is 12V), and for the 12=>5 the LM350T switching one?

[Admin]

When a motor is rated for a voltage, it means it will run at it's highest efficiency at that voltage. You can run it at other voltages no problem.

Of course, too low and the motor will be too weak to turn, and too high it might burn up.

ie, you can run your 12V motors at 15V without regulation no problem.

[waltr]

A lead acid battery's highest voltage could be 14.5V with the charger connected but will quickly drop to about 13.8 or lower once a load is connected. (check Pansonic's app notes on sealed lead acid batteries and charging).
A 12V motor is only listed at this 'nominal voltage' but will operate at a much wider range of voltages so check the motor's data sheet for the exact current draw at other voltages.

By a 'standard regulator' I guess you mean a linear regulator like the 7812 series. The problem here is that they typically have about a 3V drop so you would be required the supply 15V or more to maintain regulation at 12V out. Read the data sheet.

Powering the motors is essential because my other circuits are allready regulated.

What other circuits?

[blackietje]

What other circuits?

going to use the axon microcontrollerboard and CMUcam3 as vision sensor
I also have some servo's but they are internally powered from CMUcam3 board which has an onboard servo controller.

So your advice is just hook up the battery to the motor driver and it'll be fine?

[knossos]

Yes, you can connect the battery directly to the motor driver.  Make sure, of course, that the motor driver is capable of handling the voltage and amperage your motors require.  Also the schematic you posted would be used if you wanted to power the rest of your circuitry off the same 12v power source.

The purpose of the inductor in the circuit, as Admin pointed out, is a noise filter.  It is there to regulate the current to the voltage regulator and, as you know, inductors oppose changes in current.  If your load (the motors in this case) draws excessive current (like when the motors stall) the inductor helps to maintain a constant current to the regulator.

If you don't need a battery monitoring circuit, K1, R1 and R2 can be eliminated. 

For the LEDs and associated resistors, you can use this LED calculator to calculate appropriate resistors for your circuit.

Maybe someone with more of an engineering background can give you a better guide on capacitor selection, but these are some general things to keep in mind.  The capacitor needs to be rated for a voltage higher than the peak voltage expected, typically 1.5 x peak voltage.  If you expect a peak voltage of 15v off your 12v battery, you would need a capacitor rated for about 23v for C1.  The purpose of the capacitors in this circuit is to filter changes in voltage.  In general a larger capacitance is better able to filter low frequencies, while a smaller capacitance is better at filtering higher frequency noise.  The 220 and 470 µF capacitance values should be appropriate for a 12v power supply as well.

Lastly, I would recommend the DE 5v switching regulator.  While the LM350 can be used, it would require additional circuitry (see the datasheet) and wouldn't be a drop in replacement.

[blackietje]

Yes, you can connect the battery directly to the motor driver.  Make sure, of course, that the motor driver is capable of handling the voltage and amperage your motors require.  Also the schematic you posted would be used if you wanted to power the rest of your circuitry off the same 12v power source.

The purpose of the inductor in the circuit, as Admin pointed out, is a noise filter.  It is there to regulate the current to the voltage regulator and, as you know, inductors oppose changes in current.  If your load (the motors in this case) draws excessive current (like when the motors stall) the inductor helps to maintain a constant current to the regulator.

If you don't need a battery monitoring circuit, K1, R1 and R2 can be eliminated. 

For the LEDs and associated resistors, you can use this LED calculator to calculate appropriate resistors for your circuit.

Maybe someone with more of an engineering background can give you a better guide on capacitor selection, but these are some general things to keep in mind.  The capacitor needs to be rated for a voltage higher than the peak voltage expected, typically 1.5 x peak voltage.  If you expect a peak voltage of 15v off your 12v battery, you would need a capacitor rated for about 23v for C1.  The purpose of the capacitors in this circuit is to filter changes in voltage.  In general a larger capacitance is better able to filter low frequencies, while a smaller capacitance is better at filtering higher frequency noise.  The 220 and 470 µF capacitance values should be appropriate for a 12v power supply as well.

Lastly, I would recommend the DE 5v switching regulator.  While the LM350 can be used, it would require additional circuitry (see the datasheet) and wouldn't be a drop in replacement.

thank you for the reply knossos, i really appreciate it!

Well my motor driver's H-bridge is the LMD18200T, one for each mother and each capable of a 3A continuous output. So I guess they will do the job.
I decided to use two different power sources. One for motors (drivers) and one for my other circuitry so i can leave the 5V regulator out.

12V Lead-acid battery + battery monitor => motor drivers
7.2V Nimh batterypack => CMUcam3+Axon

So in the end i'm not going to do a power regulation which i thought was necessary

[Admin]

Well my motor driver's H-bridge is the LMD18200T, one for each mother and each capable of a 3A continuous output. So I guess they will do the job.

and

The 2 motors draw 2,8 A each and have a stall current of 6A.

Nope, not good. Given these specs, your motors will probably draw no less than 3A, meaning your drivers will overheat and burn out. You should spec your drivers to 4A or 5A, depending on how much you plan to use your motors. 6A if you plan to push your robot hard.

[blackietje]

thank you for the quick replies guys

I'm going to try those LMD18200T and see if they'll do, i'm not planning on pushing him that hard.

this is straight from it's datasheet:

Peak Output Current (200 ms) 6A
Continuous Output Current (Note 2) 3A

It's not for a racing competition or time critical application so the motor's won't be driven to the max. I'll adapt to 5A bridges if i see they still draw to much current. Any ideas on what type of solid state h-bridge i could use? Are their dual bridge drivers out there. So 2 h-bridges in one package?

thank you

[Admin]

Ok, those specs look much better. See how much they overheat . . . you may be able to get away with it, or just get a small fan to air cool them. You can also stack them on top of each other (parallel) to double the current.

If you want a professional motor driver, I'd recommend this:
http://www.dimensionengineering.com/Sabertooth2X5.htm

If you plan to maybe build something even more powerful in the future and want to reuse parts, you might just want to get this:
http://dimensionengineering.com/Sabertooth2X25.htm

[blackietje]

ok thank you all for the great help

I'll make a post when i finish the motor driver and i'll post some updates on how hot the chillis get, put some heatsinks or fans on if it goes in very hot babe mode

greetz

[Soeren]

Hi,

I'm going to try those LMD18200T and see if they'll do, i'm not planning on pushing him that hard.

You'll need to use the current sense with a little external circuitry to keep it safe (for both the chips and the motors in case you drive it onto an obstacle.

It's not for a racing competition or time critical application so the motor's won't be driven to the max. I'll adapt to 5A bridges if i see they still draw to much current. Any ideas on what type of solid state h-bridge i could use? Are their dual bridge drivers out there. So 2 h-bridges in one package?

Yes, there are dual H-bridge drivers, but the ones you have should be quite fine, provided you add the current sense to keep them in check.

[Admin]

Assuming he is going to 'mother' over his robot like most of us do, all he needs to do is touch to see how warm it gets, no current sense needed.

[Soeren]

Hi,

Assuming he is going to 'mother' over his robot like most of us do, all he needs to do is touch to see how warm it gets, no current sense needed.

I strongly disagree!

You can melt down a motor in a few seconds and the chips might go faster than the thermal inertia of the housing will allow the outside to heat up enough to know there's an issue.

All robots ought to have current sense circuitry - it's bad engineering to leave out such a small circuit with such protective potential - plain and simple.

Build it right and you don't have to baby it - it will be a grown up from the first spin 

[Admin]

Well, if it was me, I'd just over-spec the motor driver

- more circuits/code equals more possible points of failure
- less work
- adds in a comfortable safety factor margin

But yea, as long as he tests it 'in the lab' (ie with a multi-meter), and doesn't push the bot to do anything that wasn't already tested, it'll be fine . . .

[blackietje]

So i need to implement a routine for the current sense pin to shut down the system when drawing to much current?
So i should connect the pin 8 with suitable resistor to one of my analog input pin of my microcontroller and let him stop sending the PWM signal when the voltage peaks to a value to high?


Well i'm going to use these LMD18200T H-bridges (allready have them) first in order to test if it works, buying a motor driver is going to make my project even more expensive.
My school isn't going to keep paying for extra parts, only when i see i can't really do without a pre-made motor driver (will kost about 65€)
i have allready had to buy;

NimH battery
lead acid battery
chargers for both batteries
CMUcam3
Pan/tilt Kit
Axon
test breadboard

The following text i found in my LMD18200 datasheet:

USING THE CURRENT SENSE OUTPUT:

The CURRENT SENSE output (pin has a sensitivity of
377 μA per ampere of output current. For optimal accuracy
and linearity of this signal, the value of voltage generating resistor
between pin 8 and ground should be chosen to limit
the maximum voltage developed at pin 8 to 5V, or less. The
maximum voltage compliance is 12V.
It should be noted that the recirculating currents (free wheeling
currents) are ignored by the current sense circuitry.
Therefore, only the currents in the upper sourcing outputs
are sensed.

CURRENT LIMITING
Current limiting protection circuitry has been incorporated
into the design of the LMD18200. With any power device it is
important to consider the effects of the substantial surge currents
through the device that may occur as a result of
shorted loads. The protection circuitry monitors this increase
in current (the threshold is set to approximately 10 Amps)
and shuts off the power device as quickly as possible in the
event of an overload condition. In a typical motor driving application
the most common overload faults are caused by
shorted motor windings and locked rotors. Under these conditions
the inductance of the motor (as well as any series inductance
in the VCC supply line) serves to reduce the magnitude
of a current surge to a safe level for the LMD18200.
Once the device is shut down, the control circuitry will periodically
try to turn the power device back on. This feature allows
the immediate return to normal operation in the event
that the fault condition has been removed. While the fault remains
however, the device will cycle in and out of thermal
shutdown. This can create voltage transients on the VCC
supply line and therefore proper supply bypassing techniques
are required.
The most severe condition for any power device is a direct,
hard-wired (“screwdriver”) long term short from an output to
ground. This condition can generate a surge of current
through the power device on the order of 15 Amps and require
the die and package to dissipate up to 500 Watts of
power for the short time required for the protection circuitry
to shut off the power device. This energy can be destructive,
particularly at higher operating voltages (>30V) so some
precautions are in order. Proper heat sink design is essential
and it is normally necessary to heat sink the VCC supply pin
(pin 6) with 1 square inch of copper on the PCB.

[Admin]

Set up the current sensor as analog, and just tell the PWM to slow down as it reaches too high of a current.

Current spikes when you first start running the motors, so you can't just shut the motors off when current gets too high.

But again, I wouldn't use a current sensor. I'd measure the current with a multimeter to find the maximum you can bring your PWM before current gets too high. Much simpler.

[madsci1016]

Count me as a second vote for monitoring current.

My SAGAR robot has current monitoring, originally because my motors stall current would overheat the H-Bridge. Now I have an upgraded bridge (30 Amp limit) and 4 motors with 5 Amp stall rating. Though my Li-Ion batteries will safety shutdown at 12 Amps, so I still have to limit current, since the motors could trip my batteries. And this is all at 14.4V, ie a lot of power.

At first, if current went to high, i'd limit PWM until either the current dropped to a safe level, or the requested PWM dropped. Now, I'm employing fancy constant current PID controllers when the current gets too high.

Current spikes when you first start running the motors, so you can't just shut the motors off when current gets too high.

You can avoid this by using a rate limiter to apply a 'ramp up' instead of a hard 'on' (TWSS). On my PID controller, I bounded how big the change applied to PWM duty cycle could be.